In mixing and pumping cement for the oil drilling and production industry, centrifugal pumps are used for low-pressure pumping of cement slurry. These pumps may be direct driven by using a driveline that runs from a transmission mounted or engine mounted power takeoff to the pump shaft. In other applications, the pumps are driven electrically by mounting an electric motor directly to the pump frame, or the pumps may be driven hydraulically using a hydraulic pump mounted to a power takeoff that transmits power to a hydraulic motor mounted directly to the centrifugal pump.
Each mode of power transmission has advantages and disadvantages. Direct drive systems benefit from high efficiency, simplicity and relatively low weight, although driveline angle restrictions limit where the driven loads may be placed. Electric drive systems provide smooth, quiet operation but such systems are heavy and require a source of substantial electrical power. Hydraulic drive systems are lighter than electric drive systems and provide greater flexibility in load placement and orientation, but they can be vulnerable to oil contamination and other potential problems.
A conventional oilfield cementing unit with fail-safe capability typically employs full redundancy of all components important to operation. For example, if a prime mover, two centrifugal pumps and a triplex pump are required to mix and pump cement in a given cementing unit design, then the conventional redundant, fail-safe system employs two prime movers, four centrifugal pumps, and two triplex pumps. Commonly, each of the centrifugal pumps is direct-driven from a power takeoff and each power takeoff is dedicated to the particular centrifugal pump. The fully redundant system may be overly conservative because it is unlikely that of two operating centrifugal pumps, both would fail within the same job and thereby require both backup pumps to be utilized. Furthermore, the fully redundant system may present new reliability risks that are not present in a non-redundant system due to, for example, damage to or plugging of the additional piping required to plumb the backup pumps into the cementing system.
Driveline systems are known in which a power takeoff drives exactly one output without the ability to exchange pump loads between power sources. The placement and orientation of the pumps are limited by the driveline angle, and the path of the driveline limits the options for placement of major components. Sometimes, right-angle gearbox systems are employed in conjunction with drivelines to increase the number of locations in which the pumps may be placed. However, the additional gearbox adds a failure point, reduces the overall drivetrain reliability and efficiency, and creates an additional need for a gearbox lubrication and cooling system, thus increasing system complexity.
Additionally, closed-loop systems have been employed between a power source and a hydraulic pump. However, existing closed-loop systems do not work well in redundant systems because of the lack of system isolation and because of the additional components and complexity of such systems. In some applications, close-coupled hydraulic systems are employed in which a closed-loop hydraulic pump and a motor are mounted together both mechanically and hydraulically. However, such approaches provide no option for switching between different loads. Open-loop hydraulic systems also have been employed in various applications, however open-loop systems typically require hydraulic reservoirs that are significantly larger than those for closed-loop hydraulic systems.